The present invention relates to an analog signal processing technique and, more particularly, to a technique which is especially effective when applied to an analog filter circuit using a differential amplifier whose transconductance is controllable. This technique is effective when applied to a semiconductor integrated circuit device (hereinafter also referred to as an LSI) having an analog filter circuit therein and used in a device such as a portable telephone operating on a low voltage.
In recent years, as portable telephones and battery-operated electronic devices (e.g., notebook computers and cassette tape recorders) have become popular, a demand of lowering the voltage and reducing the power consumption of an LSI has been rising so as to further reduce the size and weight of the devices and to prolong the service life of the device with small-sized batteries.
On the other hand, digitalization has become more prevalent in the communications and audio fields. Since, however, the voices, images and electric waves processed are analog signals, analog circuits are indispensable. In signal processing for digital communications, moreover, A-D converters, D-A converters and analog filters, which are disposed before and after the converters so as to remove aliasing noises and high-frequency components are required.